IR radiation detectors provide an electrical output which is a measure of incident IR radiation. One particularly useful IR detector is a photovoltaic (PV) detector fabricated from Group II-VI radiation absorbing semiconductor material, such as mercury-cadmium-telluride (HgCdTe). HgCdTe detectors are typically fabricated as linear and two-dimensional arrays. Generally, a transparent substrate supports a radiation absorbing semiconductor layer having a first electrical conductivity, and a second semiconductor layer of opposite electrical conductivity forms a p-n junction with the first layer. The array may be differentiated into a plurality of p-n junctions by selectively etching the semiconductor material, resulting in the formation of a plurality of upstanding "mesa" structures, each of which contains a radiation detecting element, or pixel. The array typically includes a layer of passivation applied to an outer surface so as to reduce surface states and resulting noise signals that detrimentally affect the operation of the p-n junction. An anti-reflection (AR) coating may also be applied over the passivation layer to reduce reflections of incident radiation. Each of the radiation detecting elements includes a contact, normally provided in the form of one or more square or round metal pads. The contact pad(s) provides electrical contact for external read-out circuitry, typically via an indium "bump" interconnect, to the p-n junction. If radiation enters the array through the bottom surface of the substrate, that is, the surface opposite to the surface that supports the radiation absorbing semiconductor layer, the array is referred to as a "backside-illuminated" array.
It is conventional practice to scan incident radiation over the array, particularly linear arrays, with a rotating mirror or the like. The direction of the scanned radiation is referred to as a scan axis, and a cross-scan axis is defined to be an axis perpendicular to the scan axis. It is also conventional practice to tilt or rotate the detector array about an axis such that a radiation receiving surface of the array is inclined at an angle to incident radiation. This rotation of the array may be accomplished for both scanned and unscanned, or "staring", detector arrays
A problem that is presented during the use of such detector arrays results from reflection of radiation from the edges of the contact pads, mesa sidewall surfaces, and the like. This reflected radiation is radiation that first passes unabsorbed through the substrate, the radiation absorbing semiconductor layer, and the overlying layer of opposite conductivity. This unabsorbed radiation eventually encounters the array "top-side" edges and features and is reflected therefrom back through the body of the array. If the reflected radiation is not absorbed during the second pass through the array, the radiation emerges from the bottom surface of the substrate and may propagate back into space. This propagating radiation signal is often referred to as a "light signature".
It has been determined that one significant contributor to the light signature is the reflection from the contact pad surfaces and edges. This is due to the less than optimum shape and size, so far as minimizing reflections, that are presented by the round or square pad geometries of conventional radiation detectors.
A disadvantage of reducing the size of contact pad structures is that less than an optimal amount of area of the contact pad may contact the underlying detector. This results in poor contact resistance characteristics. This problem becomes especially apparent if it is desired to reduce the area of the square or round contact pads so as to reduce the reflected light signature. That is, as the contact pad area is reduced the contact resistance is generally increased.
What is thus an object of the invention to provide is a radiation detector having a contact pad geometry that, in combination with an adjacent radiation absorptive layer, minimizes the contact pad contribution to the light signature, while also reducing contact pad resistance.